(Supported by CNRS grants to M. Bornens). Glutamylation represents the major post-translational modification of brain tubulin. This modification is also detected in non-neuronal cell lines, but appears to be almost restricted to centrioles. Using a monoclonal antibody directed against this modification (GT335), Dr. Bornens conducted microinjection and electropermeabilization experiments in order to specifically label with centriolar tubulin. Twenty-four hours after loading HeLa cells, they failed to detect any centrosomal or centriolar marker. Short-term microtubule re-growth experiments confirmed the loss of a functional centrosome. Centrosome disappearance took place within the first 12 hours, and was maintained during 48 hours, but was reversible. During the acentrosomal period, cells were still able to progress through mitosis. These results were confirmed by two double-tilt tomographic reconstructions made on the IVEM. Thus, centrioles act as centrosome organiz ers, and the polyglutamate residues of microtubules triplets are likely involved in centriole stability. This reversible experimental system could help understanding the principles underlying centrosome stability and continuity.